This looks like model issue but since noone else has this problem I would say it is something on your side. I would try to "verify local cache intergrity"1) In your Steam Library right-click on "CoH:BK mod" and select "Properties"2) Go to "Local Files" tab and click on this button:

then click on "close", if there are some corrupted files, Steam should download the broken ones again.If this does not work, I would try to run the game without the historical addon and see if the bug is still there, if not then the problem is caused by the addon. If the addon does not cause this, I would try reinstalling the mod.

You were right, it was the D-DAY JUNE 1944 & AFRIKA 43 Historical Add-on. I had version 2.4.1 installed. I uninstalled it and now the Granatbuchse 39 works well. I just saw that there is a new version 2.4.3.